1. Field of the Invention
The present disclosure relates generally to cardiac pacing and defibrillation leads, and more particularly, to a multifilar conductor for cardiac leads.
2. Background of the Related Art
It is well known in the field of cardiology that certain types of cardiac arrhythmia can be effectively treated by the application of electrical energy to cardiac tissue in an attempt to restore a normal sinus rhythm. Endocardial leads implanted within the heart have been developed to monitor the cardiac state and automatically deliver electrical energy to cardiac tissue. These leads sense the intrinsic rhythm, atrial and ventricular tachycardia and atrial and ventricular fibrillation/flutter.
As used herein, the term ventricular tachycardia refers to any abnormally rapid heart rate (120–180 beats per minute) originating in the ventricles which is generally regular in periodicity and oftentimes is life threatening to the patient. Ventricular fibrillation/flutter is generally a more rapid heartbeat disorder, disorganized and irregular, or non-periodic, and is fatal unless corrected within minutes. Atrial tachycardia and fibrillation/flutter refers to similar abnormal behavior in the atria.
Cardioversion refers to the discharge of electrical energy into the cardiac tissue which may range from a high (40 Joules or more) to a low (less than 1 Joule) energy discharge, but is usually used to describe low energy discharges, typically delivered in the atrium, in an attempt to terminate or revert a tachycardia. Defibrillation usually refers to higher energy electrical discharges, typically delivered to the ventricles, for treating cardiac fibrillation/flutter.
Some leads are designed for defibrillation or pacing but many implantable leads are advantageously fitted for both pacing and defibrillation functions. The typical implantable lead of either arrangement is generally elongated and cylindrical in shape. Thus, for purposes of describing its features, the lead defines two opposing end portions. One end portion (hereinafter referred to as the “distal end”), has various electrodes disposed thereon for sensing heart activity and delivering electrical energy to cardiac tissue. This end is surgically placed adjacent to an inside wall of the heart and is secured at this location either actively with a fixation screw or passively with flexible tines. The other end portion (hereinafter referred to as the “proximal end”) is connected by one or more connectors with an implantable device, such as a pacemaker or defibrillator, for monitoring the distal end electrodes and supplying electrical energy.
Under normal conditions, the distal end electrodes are used by the implantable device to monitor the intrinsic electrical activity within the heart. If the implantable device senses abnormal electrical activity, such as that which results from bradycardia, tachycardia or fibrillation, it will respond by directing the appropriate amount of electrical energy to the lead to be discharged by whichever distal end electrodes are necessary to treat the abnormal cardiac activity. Thus, the proper operation of the endocardial lead depends largely on the integrity of electrical communication through the lead.
Multifilar coils constructed of helically-wound electrically conductive elements are currently commonly used to convey electrical current through endocardial leads. The conductive elements, or filars, in the multifilar coil are typically constructed from a conductive low resistance material such as MP35N, Elgiloy® or DFT. The multifilar coil gives the advantage of being highly flexible, yet resistant to breakage. Therefore, it is the current standard practice that all permanent implantable leads use coil conductors, usually having 2, 3 or 4 filars, as conveyers of electrical current through the lead. The distal end electrodes may have singular purpose, such as defibrillators or sensors, or the electrodes may have dual functions, such as pacing/sensing or sensing/defibrillation. Depending on the arrangement and amount of electrodes, the leads may have poles requiring one or more sets of anodes and cathodes. However, defibrillation and pacing functions are not usually combined in one electrode. Pacing involves low voltage discharges which have maximum effect when discharged from small surface areas. Defibrillation involves high voltage discharges which are best delivered by electrodes having larger surface areas to avoid possible tissue damage at the electrode interface and higher impedance at the area of discharge.
Current endocardial leads employ various methods for supplying the necessary electricity to the distal end electrodes. One design incorporates two coil conductors of differing diameter arranged coaxially within the lead body to provide the necessary number of electrodes for the components at the distal end. Although the coaxial arrangement may provide enough electricity for multiple components at the distal end, the configuration also possesses considerable disadvantages. For example, to provide electrical integrity between the coils, the inner and outer coil must be insulated relative to one another by an additional nonconductive covering. If the covering is compromised, which is especially possible for long-term lead implantation, electrical communication between vital components will be compromised. Also, the coaxial design increases the diameter of the lead, which may ultimately render potential applications risky or make incorporating all the desired components impossible.
Another lead design currently used incorporates multiple lumens wherein one or more lumens contain coil conductors while another one or more lumens contain low resistance stranded cable. Typically, the coil conductors connect with pacing electrodes while the stranded cables connect with defibrillation electrodes. Although this configuration reduces the potential for electrical crossover between conductors, it increases the space needed for implantation of the lead and incorporates the inherently less reliable stranded cable which may break due to fatigue or movement of the lead.
A single axis coated wire design has also been used to accommodate the need for multiple components at the distal end. In this design, each filar in a multifilar coil is individually insulated from one another and wound together. Thus, this design can accommodate multiple electrodes with just one multifilar coil without significantly increasing the profile (i.e., diameter) of the lead. However, this configuration also has considerable disadvantages. There is an increased possibility of insulation breakage, especially for long-term lead implantation because the insulation around each individual filar must be extremely thin so that the filars may be wound together. Insulation breakage may result in current leaks and voltage jumps, especially when used for defibrillation. Also, leads with this configuration have a tendency to stretch, which is particularly dangerous if an implanted lead has to be removed.
A general consideration for all leads that incorporate multifilar conductor coils relates to the increase of electrical impedance due to the winding of the filars into a coil configuration. The multifilar coil typically extends through the entire length of the lead body and requires electricity to travel through relatively long wires. The significant increase in the electrical path results in a significant increase in the electrical resistance associated with the wire material. Additionally, since the lead is designed for use with an implantable device, electricity delivered to the lead would be generated via battery power. Therefore, any increase in resistance reduces electrical efficiency and the life of the battery within the implanted device.
It would be advantageous therefore to provide a multifilar conductor coil suitable for incorporation in a single lumen implantable endocardial lead which is reliable and reduces the risk of current leakage, voltage jumps, and loss of electrical integrity. The conductor should neither significantly increase the lead diameter nor stretch during lead implantation and removal. Furthermore, the conductor should minimize the total electrical impedance so as to allow a large current to pass through the lead.